1. Field of the Invention
The present invention relates to a packet network for communicating information in forms of packets in ATM (asynchronous transfer mode) in which each packet has a fixed packet length.
2. Description of the Background Art
In a conventional packet network, a communication resource of the network has been shared on an equal basis by all of the terminals of the network, so as to facilitate an efficient utilization of the communication resource by means of an effect of statistical multiplexing due to a sharing of a single communication resource among a plurality of communications.
Namely, as shown in FIG. 1, a communication resource provided by a packet exchanger 1 of a packet network is shared by classes of communications classified into a class 1 to a class N. In this case, as shown in FIG. 2, each one of individual sub-resource r.sub.i (i=1, 2,--) of the communication resource is shared by more than one classes of communications.
However, this manner of allocation of the communication resource causes a problem that qualities of service such as a packet loss rate (or a cell loss rate) and a transmission delay are uniformly deteriorated for all the classes of communications, when more than a manegeable number of packets are entered. This is a direct consequence of sharing of the communication resource among a plurality of communications on an equal basis.
On the other hand, there has been a system of packet network in which a priority level is assigned to each class of communication in accordance with a requirement for a packet loss rate, so as to facilitate a so called priority control such that a certain degree of packet loss rate can be maintained.
However, since the communication resource has been shared among communications of different priority levels, the actual packet loss rate depends on a relative amount of packets for these communications of different priority levels, so that it has been difficult to control the packet loss rate accurately.
In addition, a lack of consideration for a communication speed of each communication has been making it difficult to maintain the packet loss rate below a certain level.
As for a call set up control to be performed when a single communication resource is shared among a plurality of communications, there has been a system of packet network which achieves the call set up control as follows. Namely, as shown in FIG. 3, each one of terminals 3 sends a call set up request individually to a multiplexer 4 of the packet network which is equipped with a multiplexed line 5, and the multiplexer 4 determines whether to set up the calls requested by judging whether it is possible to guarantee the satisfactory qualities of service to all the calls requested. Here, the qualities of service to be considered are a packet loss rate and a transmission delay. Thus, each time a call set up request is received from one of the terminals 3, the multiplexer 4 makes an assessment for the packet loss rate and the transmission delay from a known communication characteristics of each of the terminals 3, and judges whether it is possible to guarantee the satisfactory qualities of service to all the calls requesting to be set up in the packet network 3. The requesting call is set up only when this criterion is satisfied. This manner of call set up control aims to achieve a more efficient utilization of the communication resource without sacrificing the qualities of service excessively.
However, conventionally, since the packet loss rate required by the calls or estimated by the multiplexer 4 is in general an average packet loss rate evaluated for a extended period of time, so that the expected level of the packet loss rate may not be obtainable temporarily, even when the call set up control as described above is adopted.
For this reason, the conventional packet network completely ignored the temporal packet loss rate or sequential loss properties such as continuous packet losses. Therefore, when the calls possess burst property in which the packets are not necessarily discarded at random, rather continuous discarding of packets takes place, and even when the call set up control is performed to maintain the packet loss rate at certain level over an extended period of time, the temporal packet loss rate, especially that due to a continuous discarding of the packets, cannot be controlled at the satisfactory level. In particular, in a case of multiplexed calls occurring in a multiplexed line configuration of FIG. 3, a number of packets to be continuously discarded becomes overwhelmingly greater for high speed calls than for low speed calls, because for the high speed calls, as many packets as being transferred during a bursty period will be discarded continuously.
Furthermore, in a packet network, a state of traffic varies with time. For this reason, in order to maintain a certain quality of service for each call, it is necessary to judge whether to set up a call or not in accordance with an estimate for the quality of service for a case of setting up a new call, estimated on a basis of a current state of traffic and an available communication resource.
An example of a conventional method to achieve this is that in which the packet loss rate is evaluated by using a burst overflow rate. In this method, it is assumed that when a sum of the maximum speeds to send out the packets exceeds the capacity of the multiplexed line, such exceeding ones are discarded, and the packet loss rate is estimated from a probability for occurrence of such a discarding which is derived from a ratio of the maximum transfer speed and an average transfer speed of each call. Now, this method of estimating the packet loss rate using the burst overflow rate is known to require an enormous amount of calculation in a case in which the packet network is to deal with numerous types of calls. This amount of calculation in fact becomes so much for a practical situation that this method itself becomes rather impractical to implement.